The present invention relates to compositions for promoting the bonding of shaped thermoplastic elastomer articles to metallic substrates. More particularly, it relates to metallic food container closures specially adapted for use in high temperature filling, sterilization, and retort processing environments, including firmly adherent thermoplastic elastomer sealing gaskets or liners.
Vessel closures for use in food containers include a closure shell formed of either metal or plastic provided with a liner or gasket on the inner facing surface of the closure shell. The liner or gasket provides an hermetic seal between the closure member and the vessel opening. In the past, poly(vinyl chloride)-based liner formulations have been used to provide closure seals. The use of poly(vinyl chloride) resin-based compound liners is currently being discouraged for a number of reasons.
Recently, poly(vinyl chloride) (PVC) resins have received adverse EPA publicity, due to incineration, land fill, and recyclability concerns. PVC resin based plastisols conventionally employed as the closure gasket or liners, now interfere with the recyclability of both the plastic closure shell and the thermoplastic polyester bottle or container.
To overcome the shortcomings with prior art PVC-based liners and gaskets, a search is currently underway to provide substitute non-PVC type liner materials. Early efforts have focused on hot melt compositions, such as those described in U.S. Pat. Nos. 4,032,492 and 4,085,186. The compositions proposed include rubbery block copolymers based on styrene and butylene or ethyl vinyl acetate copolymers employed in combination with low molecular weight hydrocarbon oils, waxes, plasticizers, and other additives. The hot melt formulations generally possess low melting or softening points ranging from 70xc2x0 to 125xc2x0 C. In some food processing and packaging environments and applications, hot filling and pasteurization conditions are frequently carried out at temperatures above 70xc2x0 to 125xc2x0 C. Moreover, in hot fill, high retort food filling operations, in addition to elevated temperatures, internal vacuums of as high as 15-26 inches of mercury (Hg) are realized, which cause problems for low temperature softening sealing or gasketing materials. The proposed hot melt compositions generally cannot maintain the compressive set values and cut-through resistance values necessary to provide satisfactory hermetic seals under these high temperature processing conditions.
An additional requirement for liner and gasketing compositions is that they must possess good to excellent adhesion to the closure substrate to minimize the gasket or compound liner movement and cut through during hot fill and retort conditioning. The maintenance of hermetic seals during processing, case packing, shipping, and prolonged storage periods are all essential to successful food packaging.
More recently, it has been proposed to employ thermoplastic elastomer products to provide hermetic sealing structures for various plastic or metallic food vessel closures. Thermoplastic elastomers are thermo-plastic processable polymer materials possessing easy processability and rubbery mechanical performance characteristics. Thermoplastic elastomers, often referred to as TPEs, possess a number of processing advantages over earlier rubber materials because thermoplastic elastomers may be extruded and molded to shape and used with little or no extra compounding, vulcanization, or heating steps and the recycling of scrap and the ability to use common plastics processing tools and methods is a distinct advantage. Thermoplastic elastomers possess satisfactory high temperature rubbery performance characteristics to be used as liner gaskets for food closures. However, they are difficult to satisfactorily bond to metal closure materials. For this reason, they have not been readily employed.
Another effort at providing non-PVC based liner and gasketing formulations has been to employ polypropylene polymers and copolymers as the liner compound or gasketing material. Adhesion of the polypropylene liner materials to metal substrates and polymer substrates also ran into some early difficulties. For example, in U.S. Pat. No. 4,034,132, it is disclosed that the adhesion of a propylene polymer to an enamel-coated metal surface such as is provided on a foil pull tab on a container opening is improved by incorporating an adhesion-promoting amount of a carboxyl modified polypropylene resin in the metal coating enamel. In U.S. Pat. No. 4,478,677, it is disclosed that the adhesion of a heat-sealed polypropylene lined aluminum foil pull tab tape strip to an enamel coated metallic surface and opening, provided with an enamel coating formulation based on an epoxy resin, an aminoplast resin and a carboxylated polypropylene resin is further improved and made satisfactory by the addition of a butene polymer, such as polyisobutylene, into the enamel coating composition prior to its application to the metal surface.
Other efforts more directly related to bonding thermoplastic elastomer gasketing materials to metal or plastic closures are described in U.S. Pat. No. 5,060,818 wherein adhesion of the TPE gasket to the closure is promoted by incorporating a low temperature melting point liquid paraffin resin and a polypropylene resin into a thermoplastic elastomer formulation prior to injection molding or shaping the elastomer for placement in the vessel closure. Paraffin-modified formulations may be suitable for low temperature packaging operations but they generally cannot be used in high temperature processing conditions because paraffin softens at temperatures of about 250xc2x0 F.
In addition to modifying the thermoplastic elastomer compositions per se, prior to molding or shaping to form the gasket or liner, U.S. Pat. No. 5,060,818 additionally states that if an epoxy phenolic type coating is applied to the surfaces of a metallic closure, the bonding of the liner to the inner side of the vessel closure may be promoted by applying a separate layer of an adhesive which contains an oxidized polyethylene resin or an acid modified olefin resin including a carboxyl modified polypropylene resin.
Unexpectedly, in view of the foregoing, it has now been discovered that the adhesion of shaped thermoplastic elastomer articles to metallic substrates is effectively improved by the addition of a carboxyl modified polypropylene resin adhesion promoter to an enamel coating composition without the need to modify the thermoplastic elastomer composition. In addition, it has been discovered that the enamel coating composition may be modified without the need for additional polyisobutylene or polybutene resin additives. Moreover, incorporating the carboxylated polypropylene adhesion promoter for TPE materials into the enamel coating composition avoids the need to apply a separate adhesive layer comprising the carboxylated polypropylene resin in order to obtain satisfactory adhesion of TPE materials.
In accordance with this invention, and to overcome the shortcomings of the prior art arrangements, it is an object of the present invention to provide lidded, stoppered, threaded, capped, or lined metallic closures for vacuum or pressure type products requiring low orders of gas or liquid permeation with a functional hermetic seal.
It is another object of the invention to provide metallic closures provided with liner structures capable of maintaining a hermetic seal under vacuum pressure, pasteurization, hot fill, and retort processing conditions.
It is a further object of the invention to provide new and improved gasketed closures which avoid the use of PVC-based materials.
It is still another object of the invention to provide closures with non-PVC based extrusion or injection processable thermoplastic elastomers which do not require post-vulcanization to impart functional hermetic sealing closure gaskets under pasteurization and sterilization conditions.
It is a further object of the present invention to provide thermoplastic elastomer-lined metallic closures exhibiting functional torque release properties.
It is still a further object of the present invention to provide metallic closures with a heat activatable enamel coating which not only promotes adhesion of functional, non-PVC based liner and gasket materials, but also provides a metallic closure exhibiting excellent product and corrosion resistance when subjected to pasteurization, sterilization, and prolonged room temperature storage conditions.
In accordance with these and other objects, the present invention provides a new and improved enamel coating composition for promoting adhesion of shaped thermoplastic elastomer articles to metallic substrates.
In accordance with this invention, the new and improved enamel coating composition comprises a solids mixture, including: (a) from about 30% to about 90% by weight of an epoxy resin; (b) from about 10% to about 70% by weight of a phenol-formaldehyde resin cross linker; and (c) from about 0.1% to about 3% by weight of a carboxyl-modified alpha-olefin polymer resin.
In accordance with an alternate aspect, the present invention additionally provides a new and improved method of bonding a shaped thermoplastic elastomer article to a metallic surface which comprises: applying an enamel coating to a metal surface of a substrate, said enamel coating containing a solids mixture of from about 30% to about 90% by weight of an epoxy resin, from about 10% to about 70% by weight of a phenol-formaldehyde resin, and from about 0.1% to about 3% by weight of a carboxyl-modified alpha-olefin polymer resin; baking the enamel coated substrate at an elevated temperature for a time sufficient to cure and harden the enamel coating composition; heat sealing a shaped thermoplastic elastomer article to the cured enamel coated metal surface; and thereafter, permitting the heat sealed assembly to cool to ambient temperatures.
In accordance with the preferred embodiment, a new and improved liner-provided vessel closure comprises a metallic vessel closure shell having an outer facing surface and an inner facing surface. The inner facing surface includes a cured enamel coating thereon comprising a cresol-formaldehyde/epoxy resin enamel coating composition and an adhesion-promoting amount of a carboxyl-modified polypropylene resin. A shaped thermoplastic elastomer liner member, which preferably comprises a styrene-ethylene-butylene-styrene block copolymer resin, is firmly adhered to said enamel coated inner surface.
In accordance with this invention, a thermoplastic elastomer article is heat sealed to a metal surface coated with an enamel coating having incorporated therein an adhesion promoting amount of a carboxyl modified polypropylene resin. The method of the present invention eliminates the need to provide additional adhesive layers interposed between the enamel coated metal substrate and the thermoplastic elastomer. It is also avoids the need to modify the thermoplastic material prior to the heat sealing step. Moreover, the present method further provides a heat curable enamel coating composition which avoids the need for added butene polymer or other resinous components.
The new and improved enamel coating formulations and metallic closures incorporating them in accordance with this invention are less expensive to produce and use than prior art compositions. In accordance with this invention, non-PVC lined metallic closures including TPE-type gaskets exhibiting very satisfactory adhesion to the metallic closure substrates are provided.
Other objects and advantages of the present will become apparent from the following Detailed Description and illustrative working Examples.
The enamel coating compositions used in the practice of the present invention are generally epoxy resin coating formulations containing a heat activatable cross-linking resin in which small amounts of a bond promoting carboxylated polypropylene resin have been incorporated.
Epoxy resins used in the preparation of the enamel coating formulation are the polymeric reaction products of polyfunctional halohydrins with polyhydric phenols having the structural formula: 
wherein X represents the number of molecules condensed. Typical polyfunctional halohydrins are epichlorohydrin, glycerol, dichlorohydrin, and the like. Typical polyhydric phenols are resorcinol and 2,2-bis(4-hydroxyphenyl)alkanes, the latter resulting from the condensation of phenols with aldehydes and ketones, including formaldehyde, acetaldehyde, propionaldehyde, acetone, methyl ethyl ketone and the like, which result in such compounds as 2,2-bis (4-hydroxyphenyl) propane and like compounds. These epoxy resins normally contain terminal epoxy groups but may contain terminal epoxy groups and terminal hydroxyl groups.
The molecular weight of the epoxy resins may be controlled by the relative proportions of the reactants as well as by the extent to which the reaction is carried out.
In the present invention, those epoxy resins which are of relatively high molecular weight are utilized in preparing the enamel coatings. Generally, epoxy resins having an average molecular weight in the range of 1400 to 6000 may be used. Preferred resins being the condensation products of epichlorohydrin and Bisphenol A, i.e., 2,2-bis(4-hydroxyphenyl)propane.
Epoxy resins are available commercially. Preferred examples are EPON(copyright) 1004 and EPON(copyright) 1007, products of Shell Chemical Company which are the condensation products of epichlorohydrin and Bisphenol A. For maximum corrosion resistance, high molecular weight epoxy resins sold commercially under the tradename EPI-REZ(copyright)565 by Celanese Corporation is especially preferred.
The heat activatable cross-linker resin component for the epoxy resin may be any resin having a polar group which is reactive with the epoxy group, for example a hydroxyl, amino or carboxyl group. For example, phenol/formaldehyde resins, urea/formaldehyde resins, melamine/formaldehyde resins, polar group-containing vinyl resins and polar group-containing acrylic resins may be used singly or in combination.
Of these curing agent resins, the phenol/formaldehyde resins, particularly cresol/aldehyde resins, containing a polynuclear polyhydric phenol are particularly preferred from the standpoint of adhesion to the substrate, barrier properties with respect to corrosive components, and processing resistance.
The phenol/aldehyde resin component (b) used may be any phenol/aldehyde resin which contains a polynuclear phenol in the resin skeleton.
In the present invention, the term xe2x80x9cpolynuclear phenolxe2x80x9d denotes a phenol having a plurality of rings in which the phenolic hydroxyl groups are bonded. Typical examples of the polynuclear phenols are dihydric phenols represented by the formula: 
wherein R represents a direct bond or a divalent bridging group. Such phenols are used conveniently for the purpose of this invention. In the dihydric phenols of formula (II), examples of the divalent bridging group R are alkylene groups of the formula xe2x80x94CR1R2xe2x80x94 (in which each of R1 and R2 is a hydrogen atom, a halogen atom, an alkyl group having not more than 4 carbon atoms, or a perhaloalkyl group), xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94SOxe2x80x94, xe2x80x94SO2xe2x80x94 and groups of the formula xe2x80x94NR3 (in which R3 is a hydrogen atom or an alkyl group having not more than 4 carbon atoms). Generally, R is preferably an alkylene group or an ether group. Suitable examples of such dihydric phenols are 2,2-bis (4-hydroxyphenyl) propane (bisphenol A), 2,2-bis (4-hydroxyphenyl) butane (bisphenol B), 1,1-bis(4-hydroxyphenyl)ethane, bis(4-hydroxyphenyl)methane (bisphenol F), 4-hydroxyphenyl ether, and p-(4-hydroxy)phenol. Bisphenol A and bisphenol B are most preferred.
The polyhydric phenol, either alone or in combination with another phenol, is condensed with formaldehyde to give a phenol/aldehyde resin. Monohydric phenols heretofore used in the production of resins of this type can all be used as a mixture of phenols. Generally, difunctional phenols of the following formula: 
wherein R4 is a hydrogen atom or an alkyl or alkoxy group having not more than 4 carbon atoms, two of the three R4""s are hydrogen atom, and one is an alkyl or alkoxy group, and R5 is a hydrogen atom or an alkyl group having not more than 4 carbon atoms, are preferred. Other phenols such as o-cresol, p-cresol, p-t-butylphenol, p-ethylphenol, 2,3-xylenol and 2,5-xylenol, singly or in combination of two or more, are most preferred. Of course, other phenols such as phenol (carbolic acid), m-cresol, m-ethylphenol, 3,5-xylenol, m-methoxyphenol, 2,4-xylenol and 2,6-xylenol may be used, as well as other difunctional phenols such as p-aminophenol, p-nonylphenol, p-phenylphenol and p-cyclohexylphenol, all of which may be used alone or in combination with the above-mentioned polynuclear phenols in the production of the phenol/aldehyde resins.
The amount of the polynuclear phenol in the phenol/aldehyde resin may be at least 10% by weight, especially at least 30% by weight, based on the entire phenol components. A combination of the polynuclear phenol (a) and the other phenols (b) in a (a):(b) weight ratio of from 98:2 to 65:35, particularly from 95:5 to 75:25, is advantageous in regard to retorting resistance.
Formaldehyde (or paraformaldehyde) is especially suitable as the aldehyde component of the phenol/aldehyde resin. Other aldehydes such as acetaldehyde, butyraldehyde and benzaldehyde may be used singly or in combination with formaldehyde. The phenol/formaldehyde resin used in this invention may be obtained by reacting the aforesaid phenol(s) and aldehyde in the presence of a basic catalyst. The amount of the aldehyde used relative to the phenol is not particularly limited, and may be any proportion generally used in the prior art. For example, the aldehyde is used in an amount of at least 1 mole, preferably 1.5 to 3.0 moles, per mole of the phenol. Even if the aldehyde is used in a proportion of less than 1 mole, no particular inconvenience is caused.
Generally, it is desirable to carry out the condensation in a suitable reaction medium, particularly an aqueous medium. Any of basic catalysts previously used for the production of phenol-formaldehyde type resins may be used as the basic catalyst and ammonia is preferred. The basic catalyst may be present in a catalytic amount, especially 0.01 to 0.5 mole %, in the reaction medium. There is no particular restriction on the condensation conditions, and generally, condensation may be effected by heating the reactants at a temperature of 80xc2x0 to 130xc2x0 C. for a period of about 1 to 10 hours.
The resulting resin may be purified by known means. For example, the reaction product is extracted and separated from the reaction medium by using a ketone, an alcohol, a hydrocarbon or a mixture thereof, and as required, washed with water to remove the unreacted compounds. Water is removed by azeotropic distillation or sedimentation. Thus, a phenol/aldehyde resin in a form miscible with the epoxy resin can be obtained.
The epoxy resin component (a) and the phenol/aldehyde resin component (b) may be used in any desired proportions, and there is no particular restriction. From the viewpoint of the retorting resistance of the coated film, it is desirable to use a paint or enamel coating including components (a) and (b) in a weight ratio of from 90:10 to 10:90, especially from 90:10 to 70:30, respectively, for forming the inside protective coating.
The carboxylated polypropylene resin which is utilized in the practice of the present invention is prepared by grafting an unsaturated dicarboxylic acid or anhydride onto an alpha-olefin backbone)using high energy radiation or a peroxy catalyst as described in British Patent 1,020,740. Unsaturated dicarboxylic acids or anhydrides which can be employed to prepare the carboxyl modified polypropylene resins include maleic, tetrahydrophthalic, fumaric, itaconic, nadic, and methylnadic acids as well as their anhydrides, (maleic anhydride being preferred.
The amount of unsaturated dicarboxylic acid or anhydride which can be grafted onto the poly(alpha olefin) backbone ranges from about 0.05 to about 10% by weight based on the total weight of the grafted polymer and preferably the amount of grafted dicarboxylic acid or anhydride ranges from about 0.1 to about 5.0%.
Carboxyl-modified polypropylene resins are preferred as the adhesion-promoting adjuvant for the present enamel coatings. The modified polypropylene resin can be of any particle size and generally has a particle size of 0.05 to 50 microns and preferably a particle size of 35 to 40 microns.
The solids content of the enamel coating compositions of the present invention are comprised of about 70 to about 90% by weight of the epoxy resin, preferably about 75 to about 85% by weight, and about 10% to about 30% of the phenol or substituted phenolformaldehyde resin, especially preferably a cresolformaldehyde resin in an amount of about 10 to 15% by weight of the resin coating, and about 0.05 to 5% by weight of the carboxyl-modified poly(alpha olefin) resin, preferably about 1.0 to about 3% by weight of the carboxylated polypropylene resin.
In preparing the enamel coating compositions of the present invention, the epoxy resin and the phenolplast resin components are dissolved in a solvent blend, such as a mixture of ketones and aromatic hydrocarbons until these components are completely dissolved.
Suitable ketones which can be employed as solvents for epoxy resin-phenolplast resin based enamel coating formulations include methyl ethyl ketone, methyl isobutyl ketone, isophorone, cyclohexanone, diacetone alcohol and diisobutyl ketone. Aromatic hydrocarbon solvents useful as solvents for the epoxy-phenolplast resin based enamel coating formulations include benzene, toluene, xylene, and commercially available aromatic naphtha mixtures, such as Solvesso 100 or 150. An example of a useful ether alcohol is butyl cellosolve and an example of a useful ether alcohol ester is cellosolve acetate.
Antioxidants and thermal stabilizers may also be incorporated in the epoxy resin-phenolplast resin formulation to inhibit oxidation of the carboxyl modified polypropylene resin during the baking and curing of the enamel coating after its application to metal surfaces. Antioxidant compounds which have been found useful in the practice of the present invention include hindered phenolic compounds such as Irganox 1010(copyright), tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl) propionate] methane, which are incorporated in the enamel coating formulations at concentrations in the range of about 0.1 to 1.0 percent by weight based on the solids content of the enamel. Lubricants, such as low molecular weight polyethylene dispersions, which are required during forming steps in container end closure manufacture may also be incorporated in the enamel composition.
Following the procedure of U.S. Pat. No. 4,478,667, the carboxylated polypropylene resin is preferably first dissolved in a hot, e.g., greater than 100xc2x0 C., organic solvent selected from aliphatic alcohols, acids and hydrocarbons containing at least 10 carbon atoms.
The carboxylated polypropylene resin is added to the organic alcohol, acid or hydrocarbon solvent at a concentration of about 1 to about 30 percent by weight and preferably about 2 to about 10 percent by weight. After the resin is added to the solvent, the mixture is heated to a temperature above 100xc2x0 C. until the resin completely dissolves in the solvent. The carboxylated polypropylene resin solution is then added to the epoxy/phenolplast resin formulation to prepare the enamel coating composition.
Preferably the carboxylated polypropylene is at temperature above 100xc2x0 C. when added to the enamel coating formulations.
Organic alcohols used to prepare solutions of the carboxylated polypropylene resin for incorporation in the. epoxy-phenolplast resin formulations to prepare the enamel coating formulations of present inventions are long chain, saturated and unsaturated, aliphatic monohydroxy alcohols having the general formula Rxe2x80x94OH where R is a straight or branched chained saturated or ethylenically unsaturated hydrocarbon group having from 10 to 30 carbon atoms and preferably from 12 to 22 carbon atoms. Illustrative alcohols are decyl alcohol, tridecyl alcohol, lauryl alcohol, tetradecyl alcohol, cetyl alcohol, oleyl alcohol, linoleyl alcohol, palmitoyl alcohol, arachidyl alcohol, stearyl alcohol, benhenyl alcohol, arachidonyl alcohol, myristoyl alcohol and mixtures of these alcohols.
Organic acids which may be used as solvents for the carboxylated polypropylene resin include saturated and ethylenically unsaturated aliphatic acids having 10 or more carbon atoms and preferably 12 to 22 carbon atoms such as the fatty acids as capric acid, lauric acid, myristic acid, palmitic acid, isostearic acid, stearic acid and arachidic acid, undecylenic acid, myristoleic acid, palmitoleic acid, oleic acid, cetoleic acid and uric acid and mixtures of these acids.
Aliphatic hydrocarbons having 10 or more carbon atoms which may be used as solvents for the carboxylated polypropylene resin include saturated hydrocarbons such as kerosene and mineral oil as well as unsaturated hydrocarbons and particularly unsaturated hydrocarbons having olefinic or ethylenic unsaturation such as undecene, tridecene and pentadecene.
The enamel compositions of this invention can be satisfactorily applied at a solids content ranging from about 20 percent to about 70 percent by weight, based on the total weight of the liquid enamel coating composition. Generally, a solids content of 30 to 50 percent by weight is preferred.
The enamel coating composition of the present invention can be satisfactorily applied by any of the conventional methods employed in the coating industry. However, for coating of sheet metal used in container manufacture, gravure or direct roller coating are preferred methods, as the desired coating weight is easily and conventionally applied in a single coat. Spraying, dipping and flow coating are also useful methods of applying the coating dispersion.
After applying the enamel coating, it is cured and hardened by heating the coated substrate at a temperature of about 350xc2x0 F. to about 500xc2x0 F. for a period of about 20 minutes to about 1 minute, the preferred conditions being 8-10 minutes at about 375xc2x0 F.
The preferred coating weight for coating metal closures is in the range of 1.0 to 6.0 milligrams of dry coating per square inch of substrate surface to provide an enamel surface to which the TPE shaped articles may be heat sealed.
The thermoplastic elastomer materials useful for forming the shaped gaskets or liners heat sealed to the enamel coated metallic closure surfaces in accordance with this invention included alloyed blends of rubbery copolymers finely dispersed in a matrix of polyolefin as a continuous phase. Illustrative alloyed blends include a polypropylene matrix including ethylene-propylene elastomers, prevulcanized butyl rubber, sold commercially under the tradename Trefsin(copyright) from Monsanto Company, ethylene-propylene-dicyclopentadiene rubber (EPDM) sold commercially under the tradenames Vistaflex(copyright) and Santoprene(copyright) from Monsanto. Other thermoplastic elastomers may include rubbery block copolymers such as triblock copolymers of the general formula ABA, where B is an elastomeric segment and A is a thermoplastic segment, and radial block copolymers of the type having a central hub and a plurality of copolymer chains emanating therefrom having the general formula AB, where B is an elastomeric segment and is attached to the hub, and A is a thermoplastic outer segment, are useable.
These copolymers are characterized by rubber-like properties similar to those of conventional rubber vulcanizates and flow properties similar to thermoplastics at temperatures above the glass transition temperature of the end blocks. The melt behavior of these compounds, with respect to shear and temperature, is similar to the behavior of conventional thermoplastics, but melt viscosities are very much higher than those of either homopolymer of the same molecular weight. Such block copolymers have been shown to exhibit a structure wherein the elastomeric and thermoplastic segments exist in separate phases. As long as the temperature is maintained below the softening point of the thermoplastic blocks, the molecules remain pinned at each end by association of the thermoplastic segments into xe2x80x9cdomainsxe2x80x9d which are connected by flexible elastomeric chains. Thus, an elastomeric network is formed with physical cross-links in the place of the chemical cross-links of vulcanizates. When heated above the glass transition temperature of the thermoplastic segments, the domains are broken up and the polymers soften and flow.
In principle, A can be any polymer normally regarded as thermoplastic, e.g. polystyrene, polymethyl methacrylate, polypropylene, etc., and B can be any polymer normally regarded as elastomeric, e.g. polyisoprene, polybutadiene, polyisobutylene, polyethylene-butylene, EPDM, etc. In addition to the choice of the blocks, two other parameters influence the physical behavior of these compounds; total molecular weight, and the relative proportion of the two types of segments present and the mechanical properties of the two types of segments present. The mechanical properties of such block copolymers are essentially unaffected by molecular weight changes, however, the viscosities are quite sensitive to total molecular weight changes and this sensitivity is particularly apparent at low shear rates. Since none of these block copolymers exhibit Newtonian viscosity behavior, it is not possible to disclose the range of viscosities of compositions useful in the process of the invention in conventional viscosity units.
Changes in the relative proportions of the thermoplastic and elastomeric segments significantly influence both the mechanical and the flow properties of these block copolymers. As an example, a triblock copolymer wherein A is polystyrene and B is polybutadiene undergoes the following changes when the percent styrene content is varied. With a 13% styrene content, the polymer behaves like an undercured conventional vulcanizate. On increasing the styrene content to 27.5%, the behavior of the polymer is closer to that of the conventional vulcanizates. At higher styrene contents (30 to 53 percent) the polymers exhibit a yield followed by drawing and then an elastic extension. At even higher styrene content (65%), a very high yield stress is followed by a short draw and immediate break. In addition, as the styrene content is increased, the viscosity of the polymer goes through a pronounced maximum and then decreases.
In the linear triblock copolymers useful in this invention, A, the thermoplastic segment, is preferably a polymerized alkenyl aromatic compound of average molecular weight within the range of about 2,000 to 30,000. Polystyrene is a preferred material, but polymethylstyrene, polyvinyl toluene, polyvinyl naphthalene, and the like may be substituted therefor. B, the elastomeric segment, is preferably a diene polymerized from starting materials selected from the class consisting of conjugated diene hydrocarbon compounds having four to eight carbon atoms. Elastomeric copolymers of ethylene with propylene may also be useful. B is preferably polybutadiene, polyisoprene, or polyethylene-butylene having an average molecular weight per segment within the range of 10,000 to 200,000. The thermoplastic segments should contribute between about 15 and 65 percent of the molecular weight of the triblock molecule, preferably between 20 and 40 percent. Methods of synthesis of triblock compounds of this type are known to those skilled in the art and many compounds of this type are commercially available from Shell Chemical Company under the tradename Kraton(copyright).
Some of these linear triblock copolymers are subject to a degree of thermal degradation when heated to temperatures above about 150xc2x0 C. in the presence of oxygen. However, this disadvantage can be avoided by heating the copolymers in an inert atmosphere. Thermal degradation is also substantially reduced by incorporating conventional antioxidants in the compositions. The presently preferred triblock copolymers are sold by Shell Chemical Company under the tradename Kraton G. These are characterized by significantly increased thermal stability and comprise between about 20 to 40 percent styrene and a middle block of a copolymer of ethylene with butylene.
Kraton G-2705(copyright), especially preferred, is a thermoplastic rubber available from the Shell Chemical Company. More specifically, it is a linear triblock copolymer with a center elastomeric block of an ethylenebutylene polymer and end block of thermoplastic polystyrene. It supplies rubbery characteristics and film strength to the composition and is more heat resistant than the other triblock molecules.
The preferred TPE materials are also compounded with a torque release improving amount of an unsaturated fatty acid amide. Especially preferred for use as torque release additives are oleylamide and erucylamide added at amounts of 1-5% by weight of the overall TPE composition. Conventional pigments such as TiO2 or fillers such as CaSO4 and fumed silicon dioxide (silica) may also be added in conventional amounts.
In accordance with this invention, the TPE gasket is bonded or formed and bonded to the cured enamel coated metallic substrate surface by high temperature extrusion, intrusion molding, injection molding, compression molding, or pre-formed gaskets may be directly bonded by heat sealing at a temperature range of about 350xc2x0 to 400xc2x0 F. Heat sealing may be accomplished by any means known to the art, such as a hot platen press or a metal jaws heated by resistance wire or by induction heating, using dwell times varying from 0.1 seconds to 5 seconds.
After the TPE gasket or liner is heat sealed and bonded to the enamel coated metal surface, the assembly is allowed to cool to ambient temperature.
Further details regarding the compositions and methods and the attendant advantages provided by the present invention will become apparent from the following illustrative working examples.